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Tracking Spring’s Early Onset

Published: April 14th, 2011

In Washington, DC, the blooming of the cherry blossoms are a sure sign that spring has arrived. Over the last 60 years, the onset of spring has been happening earlier, particularly in the Western states. Credit: Duane Romenell/flickr.

Three Things You Should Know:

1. Since the 1950s, spring has been starting earlier in the western U.S.. The onset of spring now starts, on average, about 1.5 days earlier for every decade that passes.

2. The western U.S. is also warming more quickly than the rest of North America, on average, and research shows these warmer temperatures are causing the earlier start to spring.

3. With the help of ordinary citizens across the country, researchers are tracking how different plants and animals are responding to the shifting timing of spring.

The Debrief:

Most people have their own idea of what spring is. For some, it might be when tulip stems start poking out of the ground. For others, it’s seeing the last bit of ice break away from the riverbank and float away (of course, for others still it might be the end of the Final Four or Masters tournaments, but let’s keep things focused on the natural world for now).

For those of you that do keep track of things like blooming flowers and spring runoff, you’ll know that they don’t happen on the exact same day every year. The timing varies, coming slightly earlier or later each year, depending on how warm the weather is.

In spite of the year-to-year variation, studies of several springtime phenomena show that, since the 1950s, the season has been starting progressively earlier. For example, the cherry blossoms in Washington, D.C. bloom a couple weeks earlier today than they did in 1970 (though some of this advance could be caused by the so-called "urban heat island effect"). This “spring creep” is a response to the overall warming that has been taking place across most of North America during the same period of time.

But because every plant and animal responds differently, it’s difficult for scientists to get a sense of exactly how much and how quickly spring is advancing. To cope with all the differences, Mark Schwartz, a geography professor at the University of Wisconsin-Milwaukee developed a standard by which the onset of spring can be tracked — it’s known as the "spring index."

"The idea here is that you want a definition of spring that is the same everywhere," says Toby Ault, an earth scientist from the University of Arizona in Tucson, who has used the index in some recent studies of the changing spring onset in the western U.S.. Ault explained that this index is a mathematical model designed around when the first leaf of a lilac would appear, according to how much warm weather a region has experienced in the first few months of the year.

Even thought it was modeled after the lilac, this spring index isn't limited to a single species. Rather, it helps researchers reliably estimate when spring has "struck" in almost any North American location.

Using these models, researchers have found that in North America, spring has been advancing by about 1.2 days every decade since the 1950s. So, spring is about six days earlier now than it was in the middle of the last century.

Ault in particular looked at the changes going on in the West, which has been warming faster than the global average. He found that the extra warmth in the West meant that spring is advancing there too, by the faster rate of 1.5 days every decade.

What Makes This Important?

The timing of spring is important because, as Julio Betancourt, a geoscientist with the U.S. Geological Survey, says, "spring is where all the action is."

How early spring hits determines how long the entire growing season is for plants, he explains, and when the snow at higher elevations really begins to melt. Likewise, the fact that spring is coming earlier across the U.S., and particularly in the West, is having a noticeable impact on the natural world.

"Not only are some plants blooming earlier, but snowmelt is happening sooner, which means the streamflow can come too soon for farmers," he says. And the earlier spring and its effect on snowmelt have also contributed to the increase in large Western wildfires.

The spring onset index has helped researchers pinpoint when spring hits in various parts of the country, but it is just a model, and can’t stand in for detailed observations of specific plant and animal species. "The onset of spring is in the eyes of the thing that is springing," says Betancourt.

What is really needed, he says, is a broad set of observations of how all sorts of plants and animals across the continent are behaving in response to climate change. Only with a more extensive set of records will scientists be able to help forecast what could happen to different species in the future. To achieve this "wall to wall set of observations," Betancourt and Schwartz started a national program in 2007 to encourage ordinary citizens to gather information about the plants and animals around them.

The National Phenology Network (phenology being the study of how different species interact with the seasons) lets people report on when leaves are sprouting on a local tree, or when blossoms burst in their backyard garden. The compilation of such a rich set of observations will help researchers monitor ecological responses to climate change, and make projections the years ahead.